Method for fabricating a switching device by anodization

ABSTRACT

When an anodized film is to be formed, one formation-voltage input point is used. An interconnecting ring for anodization is provided between the input point and a pattern to be anodized. The interconnecting ring is in contact with and encloses the pattern. Another interconnecting ring to which the formation-voltage input point is connected is provided around the interconnecting ring. In addition, two junction points are provided at vertically symmetric positions with respect to the pattern. The junction points are connected to the interconnecting ring. The junction points are connected to upper connection terminals and lower connection terminals of the pattern by respective thin-line groups including a plurality of thin lines. The formation voltages at respective input points at the upper connection terminals and the lower connection terminals of the pattern are made equal to each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an anodizing interconnection and to amethod for fabricating a switching device using an anodization methodwhich is used, for example, in a production process of a liquid crystaldisplay apparatus.

2. Description of the Related Art

Active matrix type liquid crystal display apparatuses include a liquidcrystal display apparatus provided with a three-terminal nonlineardevice which is typified by a thin film transistor (TFT) as a switchingdevice. Such a liquid crystal display apparatus conventionallynecessitates six or more steps for depositing thin films and performingphotolithography during the production process. As a result, theproduction process is complicated, so that the most significantchallenges are to shorten the production time and to reduce the cost.Another liquid crystal display apparatus is provided with a two-terminalnonlinear device which is set into a conductive state at a predeterminedvoltage or more as a switching device. Such a liquid crystal displayapparatus having the two-terminal nonlinear device is superior inproduction time and cost, and consequently has been explosively andintensively developed.

A known typical two-terminal nonlinear device is a MIM(Metal-Insulator-Metal) device. In a liquid crystal display apparatususing the MIM device as a switching device, a liquid crystal layer isprovided between an active matrix substrate on which pixel electrodesand MIM devices are formed, and a counter substrate on which counterelectrodes are formed. In the liquid crystal display apparatus, avoltage applied to the liquid crystal layer is abruptly changed betweenan on state and an off state, so that a high-contrast display can beperformed even in a high duty driving which is required in response totendencies to increase the screen size of the display apparatus and toincrease the resolution.

FIGS. 11 and 12 show a construction of a one-pixel portion of aconventional active matrix substrate provided with a MIM device as aswitching device. FIG. 11 is a plan view thereof and FIG. 12 is across-sectional view taken along a line A-A' in FIG. 11. In FIGS. 11 and12, on a substrate 1 a signal line 2 made of tantalum and a lowerelectrode 3 which is branched from the signal line 2 are provided. Aninsulating film 4 made of tantalum pentoxide is provided so as to coverthe lower electrode 3. On the insulating film 4 and the substrate 1, anupper electrode 5 made of titanium is provided. Thus, a MIM device 6 isconstructed. On the upper electrode 5, a pixel electrode 7 made of ITO(Indium-Tin-Oxide) or the like is provided. The upper electrode 5 iselectrically connected to the pixel electrode 7. The thus obtainedactive matrix substrate is attached to a counter substrate in such amanner that lines made of ITO or the like on the counter substrate areperpendicular to the signal lines 2, thereby forming a liquid crystalcell.

The conventional active matrix substrate having the above-describedconstruction can be fabricated, for example, by the following method.

First, a thin tantalum film which will be the signal line 2 and-thelower electrode 3 is deposited so as to have a thickness of 3000angstroms on the glass substrate 1 by sputtering or the like. The thintantalum film is patterned into a predetermined shape byphotolithography, so as to form the signal line 2 and the lowerelectrode 3. Thereafter, by anodization, the surface of the lowerelectrode 3 is anodized, so as to form an insulating film 4 made oftantalum pentoxide and having a thickness of 600 angstroms. Next,titanium which will be the upper electrode 5 is deposited so as to havea thickness of 4000 angstroms by sputtering over the entire top surfaceof the substrate on which the above-mentioned components have beenformed. The titanium is patterned into a predetermined shape byphotolithography, so as to form the upper electrode 5. In addition, atransparent conductive film made of ITO or the like is deposited andthen patterned, so as to form the pixel electrode 7.

As shown in FIG. 13, especially in the conventional anodization, in thecase of the anodizing interconnection for forming the insulating film 4,a formation-voltage input portion 8 and an anodizing interconnection 22to connection terminal portions 10 to 21 of a pattern 9 to be anodizedboth have plane shapes. The anodization is performed by using a powersource of constant current and constant voltage. As shown in FIG. 14B, aconstant-current formation (in general, 1 mA/cm² or more) is firstperformed. In the constant-current formation, the oxidation is performedwhile the formation current (I₁) is set to be constant. As shown in FIG.14A, when the voltage reaches a value corresponding to a film thickness,a constant-voltage formation is performed at the voltage (V₁) for apredetermined time period.

In the above-described conventional anodizing method, there occurs avariation in display characteristics (Vop, Co, breakdown voltage, andthe like) among various panel portions of a liquid crystal displayapparatus. This causes a problem of nonuniformity of display.

The cause resides in the anodized film. More specifically, the thicknessand the quality of the anodized film are not uniform among various panelportions. The relationship between an applied voltage V and a current Iof a general two-terminal nonlinear device is expressed in the followingequation.

    I=αVexp(β·V.sup.1/2)

    α=(nμq/d)exp(-φ/kT)

    β=(1/kT)(q.sup.3 /π.di-elect cons..sub.1 .di-elect cons..sub.0 d).sup.1/2

n: carrier density,

μ: carrier mobility,

q: charge amount of electron,

d: thickness of anodized film,

φ: trap depth,

k: Boltzmann constant

T: ambient temperature,

.di-elect cons.₁ : dielectric constant, and

.di-elect cons.₀ : dielectric constant in vacuum.

As is apparent from the above equation, if the film thickness (d) andthe film quality (n, μ, φ, and .di-elect cons.₁) are changed, the I-Vcharacteristics (the device characteristics) are changed. This resultsin the variation, i.e., the nonuniformity, of the displaycharacteristics among the various panel portions. Moreover, in theconventional anodization method, the symmetry of the device structure(the anodized film) is lost, so that the I-V characteristic of thedevice is not symmetric. As a result, phenomena such as residual imagesoccur in display conditions of the liquid crystal display apparatus. Inaddition, because of the asymmetric I-V characteristic there arises avariation in breakdown voltages depending on the voltage applyingdirections, that is, the breakdown voltage in either the positive ornegative direction is lower than that in the other direction. This alsoresults in a problem in that a device defect can easily occur.

SUMMARY OF THE INVENTION

In the method for fabricating a switching device of the invention inwhich an insulating film formed by anodization is interposed between alower electrode and an upper electrode; when the insulating film is tobe formed, the anodization is performed for a pattern to be anodized byusing an anodizing interconnection of lines which connect a plurality ofjoint points provided between a formation-voltage input point and thepattern to be anodized to the formation-voltage input point. Theanodizing interconnection of lines also connect the joint points toinput points of the pattern so that formation voltages at the respectiveinput points of the pattern are equal to each other.

According to another aspect of the invention, in the method forfabricating a switching device in which an insulating film formed byanodization is interposed between a lower electrode and an upperelectrode, when the insulating film is to be formed, the anodization isperformed for a pattern to be anodized by using a second interconnectingring provided around a first interconnecting ring for anodization whichis in contact with and encloses the pattern, and an anodizinginterconnection consisting of a plurality of lines provided forinputting a formation voltage via a junction point connected to thesecond interconnecting ring and for making potentials between thejunction points and the first interconnecting ring equal to each other.

According to another aspect of the invention, in the method forfabricating a switching device in which an insulating film formed byanodization is interposed between a lower electrode and an upperelectrode, when the insulating film is to be formed, the anodization isperformed for a pattern to be anodized by using an anodizinginterconnection including a plurality of lines for inputting a formationvoltage via a middle position on an outer circumference of the patternat which at least two junction points provided at symmetric positionswith respect to the pattern are connected to each other in the vicinityof an interconnecting ring for anodization; the interconnecting ringbeing in contact with and enclosing the pattern, and the plurality oflines being provided so that potentials between the first junction pointand the interconnecting ring are equal to each other.

In one embodiment of the invention, a formation current density in aprocess for anodizing the pattern by using the anodizing interconnectionis preferably set to be 1 mA/cm² or less.

In another embodiment of the invention, preferably during theanodization of the pattern by using the anodizing interconnection, aformation current is gradually increased for a predetermined time perioduntil a formation voltage reaches a predetermined value. After thepredetermined time period has elapsed, anodization of the pattern isperformed while keeping the predetermined formation voltage, whereby achange of the formation current in the predetermined time period along atime axis is symmetric with respect to a change of the formation currentalong the time axis after the predetermined formation voltage isreached.

According to another aspect of the invention, the anodizinginterconnection includes: a first interconnecting ring provided incontact with a pattern to be anodized, the first interconnecting ringenclosing the pattern; a second interconnecting ring provided around thefirst interconnecting ring; and a plurality of interconnecting linesconnected to the first interconnecting ring at a plurality of positionsof the second interconnecting ring which are symmetric with respect to acenter of the first interconnecting ring.

According to another aspect of the invention, the anodizinginterconnection includes: a first interconnecting ring provided incontact with a pattern to be anodized, the first interconnecting ringenclosing the pattern; a second interconnecting for connecting aplurality of junction points to each other on an outer circumference ofthe pattern, the plurality of junction points being provided atpositions which are symmetric with respect to a center of the pattern;and a plurality of interconnecting lines, connected to the firstinterconnecting ring at a plurality of positions of the secondinterconnection which are symmetric with respect to a center of thefirst interconnecting ring.

In one embodiment of the invention, the plurality of interconnectinglines are preferably radially disposed from the plurality ofcorresponding positions toward the first interconnecting ring, and theinterconnecting lines have equal resistances.

According to the invention, when an insulating film is to be formed theanodization of a pattern is performed by using an anodizinginterconnection consisting of lines which are provided so that formationvoltages at respective input points of the pattern are made equal toeach other. By changing the shape of the anodizing interconnection fromthe plane shape to a shape including lines, the paths in which theformation current flows are made stable. Therefore, in order to equalizethe wiring resistances to the pattern to be anodized, for example, linewidths may be changed. As a result, the formation voltage and theformation current applied to various portions of the pattern to beanodized are uniform, so that the thickness and the quality of theanodized film in various panel portions are uniform. Accordingly, thedevice characteristics are uniform, and the internal structure of thedevice can be symmetric between the upper electrode side and the lowerelectrode side. In addition, the panel display characteristics (Vop, Co,breakdown voltage, and the like) can be uniform.

In order to equalize the wiring resistances to the pattern to beanodized, the anodizing interconnection specifically includes a firstinterconnecting ring for anodization which encloses a pattern to beanodized, a second interconnecting ring provided around the firstinterconnecting ring, and a plurality of anodizing lines provided sothat potentials between the first interconnecting ring and the secondinterconnecting ring are equal to each other. Alternatively, theanodizing interconnection is constructed in such a manner that aformation voltage is input via a middle position on an outercircumference of the pattern at which at least two junction pointsprovided at symmetric positions with respect to the pattern areconnected to each other in the vicinity of an interconnecting ring foranodization which encloses the pattern to be anodized.

In addition, by setting the formation current density in the anodizationto be 1 (mA/cm²) or less, the metal ion concentration in the anodizedfilm is made uniform, and the internal structure of the device can besymmetric. By applying a formation current in the anodization in asymmetric manner with respect to a time elapse, the sizes of crystalgrains in the anodized film can be symmetric between the lower electrodeside and the upper electrode side, so that the internal structure of thedevice is made symmetric. Accordingly, the I-V characteristic of thedevice is symmetric, thereby eliminating residual images in the paneldisplay. In addition, there is no difference in breakdown voltagesdepending on the voltage applying directions (a breakdown voltage ineither positive or negative direction is not lower than that in theother direction), so that it is difficult for device defects to occur.

Thus, the invention described herein makes possible the advantage ofproviding a method for fabricating a switching device, by whichswitching characteristics and display characteristics of a liquidcrystal display apparatus or the like utilizing the switchingcharacteristics can be made uniform.

This and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a two-terminal nonlinear device anda pixel electrode portion for explaining a method for fabricating anactive matrix substrate in one example of the invention.

FIG. 2A is a graph showing a relationship between time and a formationvoltage in anodization according to the invention, and FIG. 2B is agraph showing a relationship between time and a formation current in theanodization according to the invention.

FIG. 3 is a diagram showing an anodizing interconnection according to amethod for fabricating a switching device in one example of theinvention.

FIG. 4 is a diagram showing an anodizing interconnection according to amethod for fabricating a switching device in another example of theinvention.

FIG. 5 is a diagram showing an anodizing interconnection according to amethod for fabricating a switching device in still another example ofthe invention.

FIG. 6 is a partially enlarged view in the vicinity of a terminalportion shown in FIG. 5.

FIG. 7 is a graph showing a distribution condition of Ta in aconventional anodized film.

FIG. 8 is a graph showing a distribution condition of Ta in an anodizedfilm according to the invention.

FIG. 9 is a diagram showing the I-V characteristic of switching devicesaccording to a prior art and the present invention.

FIG. 10 is a graph showing changes in an electric characteristic (Vop)in a long time application of formation voltage according to a prior artand the present invention.

FIG. 11 is a plan view showing a construction of a one-pixel portion ina conventional active matrix substrate.

FIG. 12 is a cross-sectional view taken along a line A-A' in the activematrix substrate shown in FIG. 11.

FIG. 13 is a diagram showing an anodizing interconnection in aconventional method for fabricating a switching device.

FIG. 14A is a graph showing a relationship between time and a formationvoltage in conventional anodization, and FIG. 14B is a graph showing arelationship between time and a formation current in the conventionalanodization.

FIG. 15 is a diagram showing a distribution of metal ions in aconventional anodized film which has been studied by the inventors ofthe present invention.

FIG. 16 is a diagram showing a distribution of particle diameters in theconventional anodized film which has been studied by the inventors ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the problems of an anodized film which is obtained by aconventional method are studied. Thereafter, examples of the inventionwill be described.

As shown in FIG. 15, in a switching device including an anodized filmobtained by a conventional method, an anodized film 25 is interposedbetween a lower electrode 23 and an upper electrode 24. In a sideportion of the anodized film 25 closer to the lower electrode 23, aregion 25a including excessive metal ions and being short of oxygen ionsis formed. In the other side portion of the anodized film 25 closer tothe upper electrode 24, a region 25b being short of metal ions andincluding excessive oxygen ions is formed. As a result, the metal ionsexhibit a predetermined concentration gradient from the region 25a tothe region 25b. The inventors of the present invention consider that theconcentration gradient occurs because the formation current density is 1(mA/cm²) or more, so that the metal ions cannot be sufficiently diffusedand moved in the oxide film during the anodization.

In addition, according to the conventional anodization method as shownin FIG. 16, the value of a formation current in the constant-voltageformation is varied, so that the growth rate of crystal is varied.First, in the side portion of the anodized film 25 closer to the lowerelectrode 23, the current density is high, so that the ion dischargeoccurs rapidly, and the growth rate of crystal is lower than thegeneration rate of new seeds. As a result, a large number of smallcrystal grains are generated so as to form a uniform and dense film 25c.In the side portion of the anodized film 25 closer to the upperelectrode 24, the current density is low, so that the ion dischargeoccurs weakly, and the growth rate of crystal is higher than thegeneration rate of new seeds. As a result, a coarse film 25d includingcoarse and large crystal grains is formed. Accordingly, the sizes ofcrystal grains in the anodized film 25 are gradually varied from theside portion closer to the lower electrode 23 to the side portion closerto the upper electrode 24.

The above-mentioned problems occur fundamentally because of thenonuniformity of the anodized film 25 in various panel portions. Morespecifically, as shown in FIG. 13, since an anodizing interconnection 22from a formation-voltage input portion 8 to connection terminal portions10 to 21 of a pattern 9 to be anodized has a plane shape, the formationcurrent in the anodization does not flow in stable paths so that thecurrent does not uniformly flow into the respective connection terminalportions 10 to 21 of the pattern 9 to be anodized. As a result, theanodization is not uniformly performed. In addition, since theresistance values in the plane-shape anodizing interconnection 22 fromthe formation-voltage input portion 8 to the respective connectionterminals 10 to 21 of the pattern 9 are not equal to each other, thereoccurs a variation in formation voltage to be applied to the respectiveconnection terminal portions 10 to 21 in the anodization. As a result,the thickness of the anodized insulating film is not uniform.

The nonuniformity in the insulating film of the anodized film 25 occursfor the following reasons. As shown in FIG. 14B, the value of theformation current is varied from the start to the end of theanodization, so that the crystal grains have different sizes between theside portion of the anodized film 25 closer to the lower electrode 23and the side portion closer to the upper electrode 24. That is, the filmquality is varied.

Hereinafter, the present invention which has been conducted in view ofthe above-mentioned studies will be described.

FIG. 1 is a cross-sectional view of a two-terminal nonlinear device anda pixel electrode portion for illustrating a method for fabricating anactive matrix substrate in one example of the invention. In FIG. 1, athin tantalum film which will be a signal line and a lower electrode 32branched from the signal line is deposited so as to have a thickness of3000 angstroms on a glass substrate 31 by sputtering or the like. Thethin tantalum film is patterned into a predetermined shape byphotolithography, so as to form the signal line and the lower electrode32. Thereafter, the surface of the lower electrode 32 is anodized by ananodization method according to the invention so as to form aninsulating film 33 having a thickness of 600 angstrom and made oftantalum pentoxide. The insulating film 33 has a uniform thickness and auniform quality.

Specifically, a pattern is anodized by using an anodizinginterconnection which will be described later. As shown in FIGS. 2A and2B; first, a formation current is gradually increased by an adjustablepower source of constant voltage until a formation voltage reaches apredetermined value V₁ as shown in FIG. 2A (a voltage corresponding tothe film thickness). When the voltage reaches the predetermined value,the anodization is performed while maintaining the predetermined voltageV₁. During time 0 to t₁, as shown in FIG. 2B, the formation currentgradually increases and the change of the formation current along thetime axis exhibits a symmetric characteristic with respect to time pointt₁ at which the anodization and increased resistance of the insulationfilm 33 is started at the constant voltage V₁. It is preferred that theformation current density in the anodization process is set to be 1mA/cm² or less. Accordingly, the concentration of metal ions in theanodized film can be uniform and the distribution of the crystal grainsize in the anodized film can be symmetric in the thickness direction.

By sputtering or the like, over the entire top surface of the substrateon which the above-described components have been formed, titanium whichwill be an upper electrode 34 is deposited so as to have a thickness of4000 angstroms. The titanium is patterned into a predetermined shape byphotolithography, so as to form the upper electrode 34. As a result, aMIM device 35 as a switching device is obtained. In the MIM device 35,the insulating film 33 which is obtained by anodization is interposedbetween the lower electrode 32 and the upper electrode 34. Then, atransparent conductive film made of ITO or the like is deposited andpatterned, so as to form a pixel electrode 36. In this way, an activematrix substrate 37 of a liquid crystal display apparatus having the MIMdevice 35 as a switching element is obtained.

The anodized film, i.e., the insulating film 33 functions as afunctional film in the MIM device 35 as the two-terminal nonlineardevice. Accordingly, if the thickness and the quality of the anodizedfilm are changed, the I-V characteristics of the device are alsochanged. If the device characteristics are changed, the panel displaycharacteristics are changed. For this reason, in order to make thedisplay characteristics uniform, it is also necessary to make the devicecharacteristics uniform. Accordingly, the formation of the anodized filmas the insulating film 33 having a uniform thickness and a uniformquality is an important process, and hence a method for forming ananodized film will be described below in detail.

FIG. 3 is a diagram showing an anodizing interconnection according to amethod for fabricating a switching device in one example of theinvention. As shown in FIG. 3, between one formation-voltage input point41 and a pattern 42 to be anodized, an interconnecting ring (a shortring) 43 for anodization is provided. The interconnecting ring 43 is incontact with and encloses the pattern 42. The pattern 42 is constitutedof a plurality of signal lines 29 which are disposed at predeterminedintervals. Both ends of each of the signal lines 29 are connected to theinterconnecting ring 43. A plurality of lower electrodes 32 areconnected to each of the signal lines 29. In the pattern 42, the lowerelectrodes 32 are arranged in a two-dimensional array. In general, thepattern 42 corresponds to a pattern required for forming one panel of aliquid crystal display apparatus.

An interconnecting ring 44 is provided so as to enclose theinterconnecting ring 43. The interconnecting ring 44 is connected to theformation-voltage input point 41. Two junction points 45a and 45b areconnected to the interconnecting ring 44 at vertically symmetricpositions with respect to the pattern 42. The respective junction points45a and 45b are coupled to the interconnecting ring 43 by thin-linegroups 58 and 59 each including a plurality of leading thin lines. Upperconnection terminals 46 to 51 and lower connection terminals 52 to 57 ofthe pattern 42 are connected to the interconnecting ring 43. Thethin-line groups 58 and 59 are radially disposed from the respectivejunction points 45a and 45b to the interconnecting ring 43, so that theformation voltages at respective input points of the upper connectionterminals 46 to 51 and the lower interconnection terminals 52 to 57 aremade to be equal to each other. In the thin-line groups 58 and 59, thethin lines which have shorter lengths toward the center thereof havenarrower widths, so that the resistance values of the respective thinlines are made equal to each other.

The interconnecting rings 43 and 44 and the thin-line groups 58 and 59are formed on the glass substrate 31 (FIG. 1). In the case where theyare formed as a metal interconnection pattern, the pattern is made ofTa, Ti, ITO, or Al. In the case where they are formed as a conductivepaste pattern, the pattern is made of a paste of Au, Ag, Cu, Al, Ni, orC. When the conductive paste is used, the anodized film is formed byusing an anodizing interconnection which also functions as a passivationfilm for a non-anodized portion of the pattern 42 to be anodized.

FIG. 4 is a diagram showing an anodizing interconnection according to amethod for fabricating a switching device in another example of theinvention. As shown in FIG. 4, between one formation-voltage input point61 and a pattern 62 to be anodized, an interconnecting ring (short ring)63 for anodization is provided. The interconnecting ring 63 is incontact with and encloses the pattern 62. Two junction points 64a and64b are provided at vertically symmetric positions with respect to thepattern 62. These two junction points 64a and 64b are connected by apattern outer circumferential interconnection 65, and a junction point66 is disposed at a position which is equally distant from the junctionpoints 64a and 64b. The junction points 64a and 64b are connected to theinterconnecting ring 63 by thin-line groups 67 and 68, respectively, sothat the potentials therebetween are equal to each other. Each of thethin-line groups 67 and 68 includes a plurality of leading thin lines.The thin-line group 67 is an interconnection between the junction point64a and terminal portions 69 to 74. The thin-line group 68 is aninterconnection between the junction point 64b and terminal portions 75to 80. The junction point 66 is connected to the junction points 64a and64b by one continuous pattern outer circumferential interconnection 65.The junction point 66 is connected to the formation-voltage inputportion 61 having a very small area by one anodizing interconnection 81.

The material for the interconnecting ring 63, the pattern outercircumferential interconnection 65, the thin-line groups 67 and 68, andthe interconnection 81 is a metal selected from a group of Ta, Ti, ITO,Al, and the like in the case of a metal interconnection pattern, or apaste selected from a group of Au, Ag, Cu, Al, Ni, C, and the like inthe case where a conductive paste is used. In the case of the conductivepaste, an anodized film is formed by the anodizing interconnection whichalso functions as a passivation film for a non-anodized portion of thepattern 62 to be anodized.

FIG. 5 is a diagram showing an anodizing interconnection according to amethod for fabricating a switching device in still another example ofthe invention. FIG. 6 is a partially enlarged view in the vicinity ofterminal portions in FIG. 5. In FIGS. 5 and 6, if a plurality ofpatterns 99 are to be anodized (in this example, six patterns 99 areanodized), an anodizing interconnection 100 connects a formation-voltageinput portion 101 to respective connection terminal portions 102-105,106-109, 110-113, 114-117, 118-121, and 122-125, so that the resistancestherebetween are equal to each other.

The material for the anodizing interconnection 100 is a metal selectedfrom a group of Ta, Ti, ITO, and the like in the case of a metalinterconnection pattern, or a paste selected from a group of Au, Ag, Cu,Al, Ni, C, and the like in the case of a conductive paste. In the caseof the conductive paste, the anodized film is formed by the anodizinginterconnection which also functions as a passivation film for anon-anodized portion of the pattern 99 to be anodized.

Results obtained by forming an anodized film by using theabove-described anodizing interconnection are shown below.

If an anodized film having a thickness of 600 angstroms is formed, thevariation in thickness of the anodized film in a panel formed by onepattern is 50 angstroms or more according to the prior art, but thevariation in thickness can be suppressed to be 30 angstroms or lessaccording to the present invention.

As for the dielectric constant of the anodized film, a variation of 3 ormore is caused with respect to the dielectric constant of 25 accordingto the prior art, but, the variation can be suppressed to be 1 or lessaccording to the present invention. Therefore, the devicecharacteristics of various panel portions are made uniform, so that thedisplay characteristics (Vop, Co breakdown voltage, and the like) aremade uniform. Since the device characteristics are uniform, thebreakdown voltage of the device is also uniform. Accordingly, the numberof devices having lower breakdown voltages is reduced, so that thenumber of defective devices can be reduced to 50% or less as comparedwith the prior art.

According to the conventional method, the distribution of Ta ions in theanodized film is not uniform as shown in FIG. 7, so that the I-Vcharacteristic of the device indicated by broken line B in FIG. 9 isasymmetric between the positive side and the negative side of thevoltage. On the other hand, according to the present invention, as shownin FIG. 8; the distribution of Ta ions in the anodized film is uniform,so that the I-V characteristic of the device indicated by solid line Cin FIG. 9 is symmetric between the positive side and the negative sideof the voltage. Accordingly, the breakdown voltages of the device on thepositive side and the negative side have little difference. That is, oneof the breakdown voltages on the positive side and the negative side isnot lower than that on the other side. As a result, the number ofdefective devices can be reduced to 50% or less as compared with theprior art.

The evaluation results of the device defects are shown below in Table 1.

                  TABLE 1                                                         ______________________________________                                        Formation current                                                                            Number of defective                                            density        devices                                                        ______________________________________                                        1.5       mA/cm.sup.2                                                                            >30                                                        1.2       mA/cm.sup.2                                                                            3˜30                                                 0.5       mA/cm.sup.2                                                                            0˜3                                                  0.15      mA/cm.sup.2                                                                            0                                                          0.02      mA/cm.sup.2                                                                            0                                                          0.002     mA/cm.sup.2                                                                            0                                                          ______________________________________                                    

It is seen from Table 1 that the lower the formation current density is,the smaller the number of defective devices becomes. The inventors ofthe invention consider the reasons for this fact are as follows. Sincethe formation of the anodized film is slowly performed, Ta ions in thefilm exist in stable conditions, so that it is difficult for the ions tomove in the film after the formation of the film. If the Ta ions move,the anodized film is damaged. This results in a device defect.

In addition, in order to evaluate the residual images in a display in apanel utilizing the device of the invention, a voltage is applied for along time. Then, the shift of the electric characteristic (Vop) betweena panel lightening portion (a pattern display portion) and a panelnon-lightening portion (a pattern non-lightening portion) is measured.According to the conventional method, there occurs a shift differencebetween a lightening portion D1 and a non-lightening portion D2 asindicated by broken line in FIG. 10, so that residual images areobserved in the display. According to the method of the invention, thereis little shift difference between a lightening portion E1 and anon-lightening portion E2 as indicated by solid line in FIG. 10, so thatresidual images are not observed in the display.

In the above-described examples of anodizing interconnections, theinterconnecting rings 43 and 63 are used. Alternatively, theinterconnecting rings 43 and 63 may be omitted. In such a case, theanodized film may be formed by using an anodizing interconnection inwhich the lines to the connection terminals 46 to 57 and 69 to 80 arecontinuously connected by the thin-line groups 58 and 59, and thethin-line groups 67 and 68, respectively.

In the above-described examples of anodizing interconnections, twojunction points 45a and 45b or two junction points 64a and 64b aredisposed at vertically symmetric positions with respect to the pattern42 and the pattern 62, respectively. Alternatively, two junction pointsmay be disposed at horizontally symmetric positions. Also, a pluralityof junction points may be provided around the pattern 42 or the pattern62.

In the above-described examples of anodizing interconnections; by usingthe source of constant current and constant voltage for the anodizinginterconnection, the formation current density in the anodizing processis set to 1 (mA/cm²) or less. More preferably, a constant-currentanodization is performed with a formation current density of 0.5(mA/cm²) or less. After a predetermined voltage (a voltage correspondingto the film thickness) is reached, a constant-voltage anodization isperformed, thereby forming an anodized film. In this case, the metal ionconcentration in the anodized film is made to be more uniform, andtherefore the internal structure of the device can be more symmetric.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A method for fabricating a switching device inwhich an insulating film formed by anodization is interposed between alower electrode and an upper electrode, the method comprising a step forforming an insulating film on a defined pattern on a substrate byanodization,wherein the anodization is performed using aninterconnection of lines which connect a plurality of joint pointsprovided between a formation-voltage input point and the pattern to beanodized to the formation-voltage input point and which connect thejoint points to input points of the pattern, and wherein formationvoltages supplied to the respective input points of the pattern aresubstantially equal to each other, and during the anodization of thepattern, an anodizing current is gradually increased for a time perioduntil an anodizing voltage reaches a chosen value, and after the timeperiod has elapsed, anodization of the pattern is performed whilemaintaining the chosen anodization voltage, so that the change of theanodizing current in the time period along a time axis is symmetric withrespect to the change of the anodizing current along the time axis afterthe chosen anodizing voltage is reached.
 2. A method for fabricating aswitching device according to claim 1, wherein an anodizing currentdensity is set to be 1 mA/cm² or less.
 3. A method for fabricating aswitching device in which an insulating film formed by anodization isinterposed between a lower electrode and an upper electrode, the methodcomprising a step for forming an insulating film on a defined pattern ona substrate by anodization,wherein the anodization is performed usingfirst and second interconnecting rings, the first interconnecting ringbeing in contact with the pattern and enclosing the pattern, the secondinterconnecting ring being provided around the first interconnectingring and being connected to a junction point, a plurality of linesconnecting the junction point to the first interconnecting ring, forinputting to the pattern an anodizing voltage via the secondinterconnecting ring and for making potentials of the junction point andthe first interconnecting ring equal to each other.
 4. A method forfabricating a switching device according to claim 3, wherein ananodizing current density is set to be 1 mA/cm² or less.
 5. A method forfabricating a switching device according to claim 3, wherein during theanodization of the pattern, an anodizing current is gradually increasedfor a time period until an anodizing voltage reaches a chosen value, andafter the time period has elapsed, anodization of the pattern isperformed while maintaining the chosen anodization voltage, so that thechange of the anodizing current in the time period along a time axis issymmetric with respect to the change of the anodizing current along thetime axis after the chosen anodizing voltage is reached.
 6. A method forfabricating a switching device in which an insulating film formed byanodization is interposed between a lower electrode and an upperelectrode, the method comprising a step for forming an insulating filmon a defined pattern on a substrate by anodization,wherein theanodization is performed using an outer circumferential interconnectionwhich interconnects at least two junction points provided at symmetricpositions with respect to the pattern, an interconnecting ring which isin contact with the pattern and encloses the pattern, and a plurality oflines connecting each junction point to the interconnecting ring, forinputting to the pattern an anodizing voltage via a middle position ofsaid outer circumferential interconnection between said junction pointsso that potentials between the junction points and the interconnectionring are equal to each other.
 7. A method for fabricating a switchingdevice according to claim 6, wherein an anodizing current density is setto be 1 mA/cm² or less.
 8. A method for fabricating a switching deviceaccording to claim 6, wherein during the anodization of the pattern, ananodizing current is gradually increased for a time period until ananodizing voltage reaches a chosen value, and after the time period haselapsed, anodization of the pattern is performed while maintaining thechosen anodization voltage, so that the change of the anodizing currentin the time period along a time axis is symmetric with respect to thechange of the anodizing current along the time axis after the chosenanodizing voltage is reached.